1. Field of the Invention
The present invention relates to a light beam scanning optical apparatus, and particularly relates to a light beam scanning optical apparatus which is used as image writing means in a laser printer, a digital copying machine or the like.
2. Description of Prior Art
There has been known a light beam scanning optical apparatus capable of changing over the image density. In Japanese Patent Laid-Open Publication No. 59-117372, it has been disclosed an apparatus in which the image density is switched over by changing the clock frequency of drivers that drive the light emitting points and by changing the rotating speed (deflecting speed) of a polygon mirror serving as an optical deflector.
Also, in U.S. Pat. No. 5,241,329, it has been disclosed an apparatus that has a plurality of light emitting points and depicts one picture element with a specified number of light beams. In this apparatus, switchover of image density is implemented by changing the number of light beams which depict one picture element. Concretely, the apparatus has five emitting points; at an image of 240 DPI, for example, all five light beams emitted from five emitting points are used to depict one picture element. In the meantime, at an image of 300 DPI, one picture element is depicted with four light beams. In this case, at the first scan, light beams emitted from the leading four light emitting points out of five depict one complete picture element in the first scanning line; at the same time, a light beam emitted from the rest one light emitting point depicts one quarter of a picture element in the second scanning line. Then, the rest three quarters of the picture element in the second scanning line are depicted with light beams emitted from the leading three light emitting points out of five at the next scan and this picture element is completed; at the same time, the light beams emitted from the rest two light emitting points depict two quarters of a picture element in the third scanning line. Further, the rest two quarters of the picture element in the third scanning line are depicted with light beams emitted from the leading two light emitting points out of five at the following scan and this picture element is completed; at the same time, the light beams emitted from the rest three light emitting points depict three quarters of a picture element in the fourth scanning line. And so forth, picture elements are depicted.
However, in the apparatus disclosed in Japanese Patent Laid-Open Publication No. 59-117372, it would take time to change the rotating speed of the optical deflector, so that the image density could not be switched over instantaneously. On this account, for switching over the image density, there has been a restriction that a warm-up period must be ensured between different pages and moreover until the rotating speed of the optical deflector is stabilized. Therefore, for example, for a page in which character images and graphic images are intermixed within the one page, it would be impossible, even if desired, to depict the character image region at low density and depict the graphic image region at high density.
Meanwhile, with the arrangement disclosed in U.S. Pat. No. 5,241,329, it would be possible to switch over the image density within one page by changing the number of light beams which depict one picture element. However, this arrangement has problems. As being explained above, since one picture element is depicted with plural light beams, there would be a case that one picture element is formed with light beams from one scan and ones from the other scan. On this account, when there appears a shift on a light beam projecting position in the direction of main and/or sub scanning between one scan and the following scan, a picture element would not be correctly depicted. Besides, in some resolution, a dividing ratio of one picture element changes from line to line. In the above-mentioned image of 300 DPI, the picture elements of the second, the third, and the fourth scanning lines are divided in the dividing ratios 1:3, 2:2, and 3:1 respectively. Consequently, quality of image deteriorates. Further, since one picture element is depicted with plural light beams, and for that, corresponding relation between each light beam (each light emitting point) and a picture element is changing at each scan, a driving control at each light emitting point and a construction of driving circuit are on a large-scale and complicated.